Light

Light is electromagnetic radiation with a wavelength that is able to be seen to the eye or, in a technical or scientific setting, electromagnetic radiation of any wavelength. The three basic dimensions of are:

Intensity (or brilliance or amplitude, professed by humans as the glow of the light),
Frequency (or wavelength, apparent by humans as the color of the light), and
Polarization (or angle of shaking and not audible by humans under ordinary circumstances)
Due to wave-particle duality, light at the same time exhibits properties of both waves and particles. The exact nature of light is one of the key questions of modern physics.

Visible electromagnetic radiation

Visible light is the piece of the electromagnetic spectrum between the frequencies of 380 THz and 750 THz. The speed, frequency, and wavelength of a wave obey the relation:

Because the speed of light in a vacuum is fixed, able to be seen light can also be characterized by its wavelength of between 400 nanometers and 800 nm.

Speed of light
Main article: velocity of light
Even though some people express of the "velocity of light", the word velocity should be kept back for vector quantities, that is, those with both magnitude and way. The speed of light is a scalar quantity, having only magnitude and no direction, and therefore speed is the correct term.

The speed of light has been measured many times, by many physicists. The best early measurement is Ole Rømer's, in 1676. By observing the motions of Jupiter and one of its moons, Io, with a get smaller, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated a speed of 227,000 kilometers per second.